Security systems often employ the use of passive infrared (PIR) sensors for detecting motion in a region. A PIR sensor comprises a lens array that divides the protected region into sectors, a PIR detector that detects from each sector heat radiating from an object, and an amplifier/threshold detection circuit for determining if the detected heat is above a threshold producing an alarm condition. As an intruder passes through the protected region, the lens array collects and focuses the intruder's heat from each sector it passes through onto the PIR detector to produce a sine wave. The frequency of the sine wave corresponds to the speed of the intruder walking through the sectors, and the amplitude of the sine wave corresponds to the amount of heat collected by the lens array onto the detector. Additionally, because the lens array collects heat from finger-like cones that get larger as the distance from the sensor increases, the frequency and the amplitude of the sine wave are dependent on the distance of the intruder from the PIR sensor and the direction in which the intruder is traveling. If the intruder is close to the PIR sensor, the frequency and amplitude are much higher than if the intruder is on the far side of the region. The amplifier/threshold detection circuit must be designed to handle the wide range of frequencies and amplitudes produced by the extreme cases, i.e. slow walks at the far end of the region and fast walks at the close ends of the region. This causes the PIR sensor to be more susceptible to noise and false alarms.
A second problem with the PIR sensors occurs when the intruder walks directly at the PIR sensor (so-called “down the throat”) rather than across the field and through the sectors of the lens array. In this case, the PIR may not detect the intruder.
An additional problem with PIR sensors is that they are designed to detect motion over a large region but are typically used in a much smaller region. This oversizing leaves the PIR sensor more vulnerable to false alarms. Typically, the PIR sensor is designed with a frequency response that balances the fast catch characteristics of up close motion with the slow catch performance needed at maximum distance. To get crisp catch in both cases leaves the unit very false alarm prone.
To alleviate the false alarm problems, dual-technology sensors have been designed that supplement PIR detectors with other detectors such as microwave detectors. The microwave detector and the PIR detector must both detect the intruder before an alarm condition is set. An alternative design is that the microwave detector output causes the threshold of the PIR threshold detection circuit to be adjusted. Both of these designs do not obviate the problem of down the throat detection because the PIR sensor will not produce a detectable signal.
It is therefore an object of the present invention to provide a security device that uses a PIR sensor and a microwave sensor for increased performance in detecting an intruder within a region without increased false alarms.
It is a further object of the present invention to provide a security device that uses the microwave sensor to determine the distance of an object within the region to adapt the frequency response of the PIR sensor for a crisp catch without higher false alarm sensitivity.
It is a further object of the present invention to provide a security device that detects an intruder walking directly towards or away from the sensor, or “down the throat”.
It is a further object of the present invention to provide a security device that can detect motion in both a larger region and a smaller region without being prone to false alarms.